ipv6: Pass fib6_result to rt6_select and find_rr_leaf

Pass fib6_result to rt6_select. Instead of returning the fib entry, it
will set f6i and nh based on the lookup.

find_rr_leaf is changed to remove the match option in favor of taking
fib6_result and having __find_rr_leaf set f6i in the result.

In the process, update fib6_info references in __find_rr_leaf to f6i names.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>